Heat treatment of rene 95 die inserts

ABSTRACT

A rigid die insert for forming and shaping a working material. The rigid die insert comprises a nickel-base superalloy, preferably Rene 95. A plurality of gamma prime particles are uniformly distributed throughout the rigid die insert, which has a Rockwell hardness Rc of between about 48 and about 52. The invention also includes a method of treating a rigid die insert comprising a nickel-base superalloy to reduce crack propagation and raise yield stress. The method comprises the steps of: providing the rigid die insert; dissolving larger gamma-prime particles in the rigid die insert; and growing additional gamma-prime particles of smaller particle size in the rigid die insert, whereby the particle size of each of the plurality of gamma-prime particles is refined, thereby reducing crack propagation and raising the yield stress of the rigid die insert. A method of refining the particle size of gamma-prime particles in a Rene 95 superalloy is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/215,601, filed Jun. 30, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to die inserts formed fromnickel-based superalloys, and, more particularly, to rigid die insertsformed from the superalloy Rene 95 and having improved resistance tocrack propagation and yield stress.

[0003] Hot forming operations require rigid dies to form and shape theworking material. The dies are exposed to extreme temperatures andpressures and can fail under a number of life-limiting conditions suchas excessive wear, cracking or heat-checking. Furthermore, diereplacement is costly in terms of material expense, machining effort andoperation downtime. Efforts to increase die life are constantly sought.To limit replacement costs, die inserts are formed from materials suchas nickel-base superalloys that can withstand the extreme conditionsencountered during hot forming. Therefore, what is needed is a rigid dieinsert, formed from a superalloy, that has improved resistance to crackpropagation and improved yield stress. What is also needed is a methodof treating rigid die inserts formed from a nickel-base superalloy thatwill increase the life of the die by improving the crack propagationresistance and yield strength of the die insert.

SUMMARY OF THE INVENTION

[0004] The present invention meets these needs and others by providing arigid die insert having an extended usable life when used in hot formingoperations. The rigid die insert is made from a nickel-base superalloy,preferably Rene 95, having fine-grained gamma-prime (γ′) particlesuniformly distributed throughout the die.

[0005] The invention also provides a heat treatment process for diesformed from a nickel-base superalloy, preferably Rene 95, to extendusable life of the die by improving the crack propagation resistance andyield strength of the die.

[0006] Accordingly, one aspect of the present invention is to provide arigid die insert for forming and shaping a working material. The rigiddie insert comprises a nickel-base superalloy, wherein a plurality ofgamma prime particles are uniformly distributed throughout the rigid dieinsert, and wherein the rigid die insert has a Rockwell hardness, R_(c),of between about 48 and about 52.

[0007] A second aspect of the present invention is to provide anickel-base superalloy for forming a rigid die insert. The nickel-basesuperalloy comprises a Rene 95 alloy and is formed by heating the Rene95 alloy to a sub-solvus temperature for a first predetermined holdtime, quenching the Rene 95 in a room temperature bath, and heating theRene 95 alloy to a second predetermined temperature for a secondpredetermined hold time. The nickel-base superalloy has a plurality ofgamma prime particles uniformly distributed throughout, and a Rockwellhardness, Rc, of between about 48 and about 52.

[0008] A third aspect of the present invention is to provide a rigid dieinsert for forming and shaping a working material. The rigid die insertcomprises a treated Rene 95 superalloy, the Rene 95 superalloy beingtreated by heating the Rene 95 superalloy to a sub-solvus temperaturefor a first predetermined hold time, quenching the Rene 95 superalloy ina room temperature bath, and heating the Rene 95 superalloy to a secondpredetermined temperature for a second predetermined hold time. Thetreated Rene 95 superalloy has a plurality of gamma prime particlesuniformly distributed throughout, and a Rockwell hardness, R_(c), ofbetween about 48 and about 52.

[0009] A fourth aspect of the present invention is to provide a methodof treating a rigid die insert to reduce crack propagation and raiseyield stress, wherein the rigid die insert comprises a nickel-basesuperalloy having a plurality of gamma-prime particles, each of thegamma-prime particles having a particle size. The method comprises thesteps of: providing the rigid die insert; dissolving larger gamma-primeparticles in the rigid die insert; and growing additional gamma-primeparticles of smaller particle size in the rigid die insert, whereby theparticle size of each of the plurality of gamma-prime particles isrefined, thereby reducing crack propagation and raising the yield stressof the rigid die insert.

[0010] A fifth aspect of the present invention is to provide a method ofrefining the particle size of gamma-prime particles in a Rene 95superalloy. The method comprises the steps of: providing a Rene 95superalloy; heating the Rene 95 superalloy to a first temperature, thefirst temperature being a temperature below a solvus temperature of theRene 95 superalloy thereby dissolving larger gamma-prime particles;quenching the Rene 95 superalloy at room temperature in a bath; andaging the Rene 95 superalloy at a second predetermined temperature for asecond predetermined hold time, thereby growing additional gamma-primeparticles of smaller particle size, whereby a more uniform sizedistribution of gamma-prime particles is created.

[0011] A sixth aspect of the present invention is to provide a method oftreating a rigid die insert to reduce crack propagation and raise yieldstress. The rigid die insert comprises a Rene 95 superalloy having aplurality of gamma-prime particles, each of the gamma-prime particleshaving a particle size. The method comprises the steps of: providing therigid die insert; heating the rigid die insert to a first temperaturefor a first predetermined hold time, the first temperature being atemperature below a solvus temperature of the Rene 95 superalloy;forced-air cooling the rigid die insert; quenching the rigid die insertat room temperature in a bath, thereby dissolving larger gamma-primeparticles; and aging the rigid die insert at a second predeterminedtemperature for a second predetermined hold time, whereby the particlesize of each of the plurality of gamma-prime particles is refined,thereby reducing crack propagation and raising the yield stress of therigid die insert.

[0012] These and other aspects, advantages, and salient features of theinvention will become apparent from the following detailed description,the accompanying drawing, example, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic representation of a die assembly having arigid die insert of the present invention; and

[0014]FIG. 2 is a plot of die lifetime for die inserts of the presentinvention versus prior-art die inserts.

DETAILED DESCRIPTION OF THE INVENTION

[0015] In the following description, like reference characters designatelike or corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” and the like are words of convenience and are notto be construed as limiting terms.

[0016] Referring to the drawings in general and to FIG. 1 in particular,it will be understood that the illustrations are for the purpose ofdescribing a preferred embodiment of the invention and are not intendedto limit the invention thereto. FIG. 1 is a schematic cross-sectionalview of a die assembly with the die insert of the present invention. Die10, which is used to hot work working material 30, typically comprisestwo half segments 12 and 22. Each half segment 12 and 22 has an outershell 14 and rigid a die insert 16. The outer shell 14 may be formedfrom a material such as, but not limited to, high strength tool steel.

[0017] The present invention provides a rigid die insert 16 having anextended usable life when used in hot forming operations. The rigid dieinsert 16 is made from a nickel-base superalloy, preferably Rene 95, andcontains a plurality of fine-grained gamma-prime (γ′) particlesuniformly distributed throughout the die. Rene 95 is a nickel-basesuperalloy comprising: between about 12 and 14 weight percent chromium;between about 7 and 9 weight percent cobalt; between about 3.3 and about3.7 weight percent molybdenum; between about 3.3 and about 3.7 weightpercent tungsten; between about 3.3 and about 3.7 weight percentniobium; between about 2.3 and about 2.7 weight percent titanium;between about 3.3 and about 2.7 weight percent aluminum; between about0.04 and about 0.09 weight percent carbon; between about 0.006 and about0.016 weight percent boron; between about 0.03 and about 0.07 weightpercent zirconium; with the balance of the alloy being nickel. Thepresent invention also provides a heat treatment process for dies madeof the superalloy Rene 95 to extend the usable life of the die 10 whenit is used in hot forming operations. The heat treatment has been shownto increase die life by over 87%.

[0018] As-received superalloy material, such as Rene 95, typicallycomprises a combination of large and small gamma prime (γ′) particles ina nickel matrix with a Rockwell hardness, R_(c), of about 42. Thecentral idea of the heat treatment of the present invention is toinhibit crack propagation and raise the yield stress by eliminatinglarge gamma prime particles and thus obtain a more uniform sizedistribution of the gamma prime particles. This is achieved throughheating the superalloy material to a sub-solvus temperature, followed byrapid cooling in a room temperature bath. This combination of heatingand quenching dissolves the large gamma prime particles and creates amore uniform distribution of fine gamma prime particles. Following thequench, additional fine gamma prime precipitate are grown from solutionby an aging process, which includes heating to a predeterminedtemperature for a specified hold time. Superalloy materials that areheat treated according to the present invention exhibit an increase inhardness. The measured Rockwell hardness, R_(c), of Rene 95 materialafter the heat treatment according to the present invention is in therange from about 48 to about 52.

[0019] The cooling, or quench, rate must be fast enough to lock in thedesirable microstructure. At the same time, however, the cooling ratemust be sufficiently slow to avoid cracking the material due to thermalgradient-induced stresses. Cracks can be initiated at the surface orcenter of the material due to the rapidly cooling material that shrinksand yields plastically in tension. These stresses must be kept below theultimate strength of the material.

[0020] In the present invention, a rigid die insert formed from anickel-base superalloy, preferably Rene 95, is heated slowly to asub-solvus temperature; i.e., below the solvus temperature of theparticular nickel-base superalloy. For rigid die inserts formed fromRene 95, the rigid die insert 16 is heated to about 2050° F. in an inertatmosphere, such as argon, and held at temperature for about 2 hours.Upon removal from oven, the rigid die insert 16 is immediately cooledwith forced air provided by a fan or any equivalent device. After aperiod of air cooling, the duration of which is governed by the beforementioned constraints on yielding, the rigid die insert 16 is quicklyquenched in a room-temperature bath. For rigid die inserts ofapproximately 4″×2″×1½″ in size, the air cooling duration is about 30seconds. An oil bath is preferably used for the quench bath. The rigiddie insert 16 is left in the bath until safe removal is possible.Finally, the rigid die insert 16 is aged by slowly heating the rigid dieinsert 16 to a second temperature for a predetermined hold time. Forrigid die inserts 16 formed from Rene 95, the aging step comprisesheating the rigid die insert 16 to about 1400° F. in an inertatmosphere, such as argon, and holding the rigid die insert 16 attemperature for about 16 hours.

[0021] The most appropriate sub-solvus temperature and cooling rate isdependent on the size and geometry of the rigid die insert 16 that isheat treated. Whereas the heat treatment of the present invention hasbeen optimized for dies of approximately 4″×2″×1½″ in size, a similarheat treatment, with minor modifications to cooling rates, isappropriate for larger or smaller dies or dies of differing geometry.

[0022] The following example serves to illustrate the advantages andfeatures of the present invention.

EXAMPLE 1

[0023] Parts were hot formed using three types of dies: a die of anearlier design having a Rene 95 die insert that was not heat treatedaccording to the present invention; a redesigned die having a Rene 95die insert that was not heat treated according to the present invention;and a redesigned die having a Rene 95 die insert that was heat treatedaccording to the present invention. The average lives of the differentdie types, represented by the average number of pieces produced beforedie failure, are compared in FIG. 2. The die of earlier design (“OldStyle Die” in FIG. 2) produced an average of 975 pieces before failure.The redesigned die which did not have a Rene 95 die insert that was heattreated according to the present invention (“New Style Die” in FIG. 2)produced an average of 1442 pieces before failure. Finally, theredesigned die having a Rene 95 die insert that was heat treatedaccording to the present invention (“New Style Die & New Heat Treat” inFIG. 2) produced an average of 2696 pieces before failure. The resultsindicate that the redesigned die inserts that were heat treatedaccording to the present invention alone increased die life by 87% overthe redesigned die having Rene 95 die inserts that were not treatedaccording to the present invention.

[0024] While various embodiments are described herein, it will beapparent from the specification that various combinations of elements,variations, or improvements thereon may be made by those skilled in theart, and are thus within the scope of the invention. For example, anentire die may be formed from a nickel-base alloy and heat treatedaccording to the method described herein.

What is claimed is:
 1. A rigid die insert for forming and shaping aworking material, said rigid die insert comprising a nickel-basesuperalloy, wherein a plurality of gamma prime particles are uniformlydistributed throughout said rigid die insert, and wherein said rigid dieinsert has a Rockwell hardness, R_(c), of between about 48 and about 52.2. The rigid die insert according to claim 1, wherein said nickel-basesuperalloy is Rene
 95. 3. A nickel-base superalloy for forming a rigiddie insert, said nickel-base superalloy comprising a Rene 95 alloy andbeing formed by heating said Rene 95 alloy to a sub-solvus temperaturein an inert atmosphere for a first predetermined hold time, quenchingsaid Rene 95 in a room temperature bath, and heating said Rene 95 alloyto a second predetermined temperature for a second predetermined holdtime in an inert atmosphere, wherein said nickel-base superalloy has aplurality of gamma prime particles uniformly distributed throughout, andwherein said nickel-base superalloy has a Rockwell hardness, R_(c), ofbetween about 48 and about
 52. 4. The nickel-base superalloy of claim 3,wherein said sub-solvus temperature is about 2050° F., and wherein saidfirst predetermined hold time is about two hours.
 5. The nickel-basesuperalloy of claim 3, wherein said second predetermined temperature isabout 1400° F., and wherein said second predetermined hold time is about16 hours.
 6. A rigid die insert for forming and shaping a workingmaterial, said rigid die insert comprising a Rene 95 superalloy, whereinsaid rigid die insert is heated in an inert atmosphere to a sub-solvustemperature of said Rene 95 superalloy for a first predetermined holdtime, quenched in a room temperature bath, and heated in an inertatmosphere to a second predetermined temperature for a secondpredetermined hold time, wherein said rigid die insert has a pluralityof gamma prime particles uniformly distributed throughout, and whereinsaid rigid die insert has a Rockwell hardness, R_(c), of between about48 and about
 52. 7. The rigid die insert of claim 6, wherein saidsub-solvus temperature is about 2050° F., and wherein said firstpredetermined hold time is about two hours.
 8. The rigid die insert ofclaim 6, wherein said second predetermined temperature is about 1400°F., and wherein said second predetermined hold time is about 16 hours.9. A method of treating a rigid die insert to reduce crack propagationand raise yield stress therein, the rigid die insert comprising anickel-base superalloy having a plurality of gamma-prime particles, eachof the gamma-prime particles having a particle size, the methodcomprising the steps of: a) providing the rigid die insert; b)dissolving larger gamma-prime particles in the rigid die insert; and c)growing additional gamma-prime particles of smaller particle size in therigid die insert, whereby the particle size of each of the plurality ofgamma-prime particles is refined, thereby reducing crack propagation andraising the yield stress of the rigid die insert.
 10. The method ofclaim 9, wherein the step of dissolving larger gamma-prime particlescomprises the steps of: a) heat treating the rigid die insert in aninert atmosphere to a first predetermined temperature for a firstpredetermined hold time, said first predetermined temperature being asub-solvus temperature; and b) quenching the rigid die insert in a roomtemperature bath.
 11. The method of claim 10, further including the stepof forced-air cooling the rigid die insert after the step of heattreating the rigid die insert to a first predetermined temperature. 12.The method of claim 10, wherein the inert atmosphere is an argonatmosphere.
 13. The method of claim 10, wherein the step of quenchingthe rigid die insert in a room temperature bath comprises quenching therigid die insert in a room temperature oil bath.
 14. The method of claim9, wherein the step of growing additional gamma-prime particles ofsmaller particle size comprises aging the rigid die insert in an inertatmosphere to a second predetermined temperature for a secondpredetermined hold time.
 15. The method of claim 14, wherein the inertatmosphere is an argon atmosphere.
 16. A method of refining the particlesize of gamma-prime particles in a Rene 95 superalloy, the methodcomprising the steps of: a) providing a Rene 95 superalloy; b) heatingthe Rene 95 superalloy in an inert atmosphere to a first temperature,the first temperature being a temperature below a solvus temperature ofthe Rene 95 superalloy; c) quenching the Rene 95 superalloy at roomtemperature in a bath, thereby dissolving larger gamma-prime particles;and d) aging the Rene 95 superalloy in an inert atmosphere at a secondpredetermined temperature for a second predetermined hold time, therebygrowing additional gamma-prime particles of smaller particle size,whereby a more uniform size distribution of gamma-prime particles iscreated.
 17. The method of claim 16, wherein the step of heating theRene 95 superalloy in an inert atmosphere to a first temperaturecomprises heating the Rene 95 superalloy to about 2050° F. for about twohours.
 18. The method of claim 16, wherein the step of quenching theRene 95 superalloy at room temperature in a bath comprises quenching theRene 95 superalloy in a room temperature oil bath.
 19. The method ofclaim 16, wherein the step of aging the Rene 95 superalloy in an inertatmosphere at a second predetermined temperature for a secondpredetermined hold time comprises heating the Rene 95 into about 1400°F. for about 16 hours.
 20. The method of claim 16, wherein the inertatmosphere is an argon atmosphere.
 21. A method of treating a rigid dieinsert to reduce crack propagation and raise yield stress, the rigid dieinsert comprising a Rene 95 superalloy having a plurality of gamma-primeparticles, each of the gamma-prime particles having a particle size, themethod comprising the steps of: a) providing the rigid die insert; b)heating the rigid die insert in an inert atmosphere to a firsttemperature for a first predetermined hold time, the first temperaturebeing a temperature below a solvus temperature of the Rene 95superalloy; c) forced-air cooling the rigid die insert; d) quenching therigid die insert at room temperature in a bath, thereby dissolvinglarger gamma-prime particles; and e) aging the rigid die insert in aninert atmosphere at a second predetermined temperature for a secondpredetermined hold time, whereby the particle size of each of theplurality of gamma-prime particles is refined, thereby reducing crackpropagation and raising the yield stress of the rigid die insert. 22.The method of claim 21, wherein the step of quenching the rigid dieinsert in a room temperature bath comprises quenching the rigid dieinsert in a room temperature oil bath.
 23. The method of claim 21,wherein the step of heating the rigid die insert in an inert atmosphereto a first temperature for a first predetermined hold time comprisesheating the rigid die insert to about 2050° F. for about two hours. 24.The method of claim 21, wherein the step of aging the rigid die insertin an inert atmosphere at a second predetermined temperature for asecond predetermined hold time comprises heating the rigid die insertinto about 1400° F. for about 16 hours.
 25. The method of claim 21,wherein the inert atmosphere is an argon atmosphere.